End-to-end connections of a data transmission system, such as a telephone network, often show long transit time delays, in consequence of which echo is observed for instance in the case of normal speech, when a signal is reflected from the far end of a connection back to the talker.
Electric echo is mostly caused by 2/4-wire transformers situated in terminal exchanges of a fixed telephone network or at remote subscribers' stages. Subscribers' lines of a fixed network generally comprise 2 wires for economical reasons. Interconnecting lines between exchanges comprise 4 wires.
An echo canceller is a device for processing a signal, such as a speech signal, so as to reduce echo by subtracting estimated echo from the echo (signal) occurring in a connection. The echo canceller can be either digital or analog. Echo cancelling devices are at present realized by digital signal processing, by means of which it is possible to model echo paths including considerably long transit time delays. Since the echo path in principle is different at each call, it is necessary to apply to an echo canceller a method which always adapts itself to a new echo path at the start of a call. Digital signal processing offers as a solution an adaptive filter, in which the coefficients of the filter modelling the echo path are updated on the basis of a correlation of the speech signal and the returning echo signal.
If a call takes place in a noisy environment, it causes certain problems with the stability of the echo path impulse response model in the echo canceller. Difficulties arise particularly if the far end is noisy and the echo level of the near end low. Divergence then occurs in the impulse response model of the echo canceller on low levels of the signal received from the far end. On the other hand, if the level of the signal received from the far end rises, echo is observed at the far end for a short time because of the diverged model of the echo path, before the echo canceller is converged back to the echo path.